What's wrong with register(_:forCellWithReuseIdentifier:) on UICollectionView?

Question

I'm working with an UICollectionView. As dequeueReusableCell(withReuseIdentifier:for:) expects You must register a class or nib file using the register(_:forCellWithReuseIdentifier:) method before calling this method, I added a line in my viewDidLoad func as

self.collectionView!.register(PhotoCollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)

Now when I'm using the cell for dequeuing and configuring, I'm getting error and app crashes.

fatal error: unexpectedly found nil while unwrapping an Optional value

This is my DataSource method:

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier,
                                                  for: indexPath) as! PhotoCollectionViewCell

    let aPhoto = photoForIndexPath(indexPath: indexPath)
    cell.backgroundColor = UIColor.white

    cell.imageView.image = aPhoto.thumbnail //this is the line causing error

    return cell
}

And this is my PhotoCollectionViewCell class

class PhotoCollectionViewCell: UICollectionViewCell {

    @IBOutlet weak var imageView: UIImageView! //I double checked this IBOutlet whether it connects with Storyboard or not
}

Original question

Now comes the interesting part.

I'm using a prototype cell in the UICollectionView and I set a reusable identifier from attributes inspector. Also I changed the custom class from identity inspector to my PhotoCollectionViewCell.

I searched for the same issue and found out that when using prototype cell, deleting

self.collectionView!.register(PhotoCollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier) 

from code will work. I gave it a try and it works.

But I'm curious to know the reason behind this issue. I couldn't reproduce the same issue with UITableView but with UICollectionView.

Not a possible duplicate:

This UICollectionView's cell registerClass in Swift is about how to register class in UICollectionView. But my question doesn't expect how to register. My question is about an issue that isn't true with UITableView class but with UICollectionView only. I'm expecting the actual difference between this conflicting issue.

Answer 1

There are 3 ways to register a cell (either for UITableView or UICollectionView).

1. You can register a class. That means the cell has no nib or storyboard. No UI will be loaded and no outlets connected, only the class initializer is called automatically.

2. You can register a UINib (that is, a xib file). In that case the cell UI is loaded from the xib and outlets are connected.

3. You can create a prototype cell in the storyboard. In that case the cell is registered automatically for that specific UITableViewController or UICollectionViewController. The cell is not accessible in any other controller.

The options cannot be combined. If you have a cell prototype in the storyboard, you don't have to register again and if you do, you will break the storyboard connection.